Multiple methods exist for measuring porosity. A common method for measuring porosity is to calculate porosity from measurements of both bulk density, also referred to as envelope density, and skeletal density, also referred to as grain density. This technique is commonly used in laboratory core analysis, bulk density measured by caliper and skeletal density measured by pycnometry; and in downhole logging, bulk density measured by a density tool and skeletal density assumed or estimated from mineralogy. Porosity is also commonly measured downhole with a neutron tool. Finally, porosity may be measured by nuclear magnetic resonance (NMR) both downhole, for example with the Schlumberger's Combinable Magnetic Resonance Tool (CMR tool), and on cores or cuttings.